Enhancing Förster Resonance Energy Transfer (FRET) Efficiency of Titania-Lanthanide Hybrid Upconversion Nanomaterials by Shortening the Donor-Acceptor Distance.

Several robust titania (TiO2) coated core/multishell trivalent lanthanide (Ln) upconversion nanoparticles (UCNPs) hybrid architecture designs have been reported for use in photodynamic therapy (PDT) against cancer, utilizing the near-infrared (NIR) excited energy down-shifting and up-conversion chain of Nd3+ (λ793-808 nm) → Yb3+ (λ980 nm) → Tm3+(λ475 nm) → TiO2 to produce reactive oxygen species (ROS) for deep tissue-penetrating oxidative cytotoxicity, e.g., NaLnF4:Yb,Tm (Ln = Y, Gd). Herein, we demonstrate that by doping the Tm3+ emitter ions in the outer shell and the Nd3+ sensitizer ions in the core, the newly designed NaYF4:Nd,Yb@Yb@Yb,Tm@TiO2 hybrid UCNPs exert more ROS production than the reference NaYF4:Yb,Tm@Yb@Nd,Yb@ TiO2 with the Tm3+ ions in the core and the Nd3+ ions in the outer shell, upon 793 nm laser irradiation, primarily due to the shortening of the Tm3+-TiO2 distance of the former with greater Förster resonance energy transfer (FRET) efficiency. After coating with polyallylamine hydrochloride (PAH)/polyethylene glycol folate (PEG-FA), the resulting NaYF4:Nd,Yb@Yb@Yb,Tm@TiO2-PAH-PEG-FA hybrid nanocomposites could be internalized in MDA-MB-231 cancer cells, which also show low dark cytotoxicity and effective photocytotoxicity upon 793 nm excitation. These nanocomposites could be further optimized and are potentially good candidates as nanotheranostics, as well as for other light-conversion applications.